A multifocal lens is constituted of a principal lens in glass and of one or more segments in glass which are fused onto it. These segments have a higher refractive index than that of the principal lens; this enables obtaining areas of higher corrective power. Said segment(s) is (are) normally soldered onto the principal lens during a heat treatment bringing the whole of the glasses to a temperature close to the softening point of the glass constituting the principal lens. Consequently, the segment glasses must have characteristics of viscosity close to those of the glass of the principal lens. It must also have close thermal expansion characteristics, so that the resulting soldering has a very low strain level.
The objective of the present invention was to obtain glasses of refractive index between 1.70 and 1.78, fusible particularly to an ophthalmic glass manufactured by the Applicant Company (Corning S.A.). of index 1.6. of Abbe number 41 and which has the following physical characteristics:
thermal expansion coefficient: 100.times.10.sup.-7 K.sup.-1 (between 25 and 300.degree. C.), PA1 softening point: 610.degree. C. PA1 strain point: 460.degree. C., PA1 annealing point: 490.degree. C. PA1 a low tendency to devitrify and a high liquidus viscosity (.gtoreq.10 Pa.s); which facilitates their forming and limits their costs of production; and PA1 as much as possible, a good chemical durability. PA1 high refractive index: 1.70.ltoreq.n.ltoreq.1.78; PA1 thermal expansion coefficient: between about 90 and 110.times.10.sup.-7 K.sup.-1 (between 25 and 300.degree. C.); PA1 softening point: between about 570 and 620.degree. C. PA1 low tendency to devitrify and high liquidus viscosity (.gtoreq.10 Pa.s).
Said glass of the Applicant Company bears the Code-name 8008. It was described in U.S. Pat. No. 4,540,672.
It has been determined experimentally that, in order to be fusible to said glass Code-named 8008, the glasses of the invention must have a thermal expansion coefficient between about 90 and 110.times.10.sup.-7 K.sup.-1 and a softening point between about 570 and 620.degree. C.
Furthermore, it was desired that said glasses of the invention have:
The technical problem to be solved within the context of the present invention has therefore been that of providing glasses having the specifications below:
Within the context of an advantageous variant of the invention, it was sought that said glasses, having the specifications above, have furthermore a good chemical durability.